1. Field of the Invention
The present invention relates to a planar antenna apparatus, such as a wideband antenna apparatus, capable of being used in the fields of high precision positional detecting techniques, large capacity fast signal transmission techniques, and the like.
2. Description of the Related Background Art
Conventionally, there has been proposed a planar type antenna apparatus in which a co-planar waveguide 1 is formed on a planar substrate, and a center conductor 1a of the co-planar waveguide 1 is shaped into a T-shape at its end portion, as illustrated in FIG. 26. In FIG. 26, reference numeral 1b designates a grounded conductor, reference numeral 2 designates a slot, reference numeral 3 designates electric fields, and reference numeral 4 designates a short-circuit line. In the antenna apparatus illustrated in FIG. 26, resonance occurs at a frequency whose half wavelength is equal to the length of the T-shaped conductor (see Japanese Patent Laid-Open No. 1 (1989)-300701; Reference 1).
Further, in recent years, in tandem with the high precision positional detecting techniques and large capacity fast signal transmission techniques, ultra wideband (UWB) techniques using a wide frequency region in a range from 3.1 GHz to 10.6 GHz have been energetically developed. When such a wide frequency region is used, the time resolution of a pulse can be improved in positional detecting techniques using a pulse radar, for example, thus allowing high precision positional detection to be achieved.
In connection with signal transmission techniques, usable band width can be widened, and accordingly the throughput of signals is expected to increase.
As an antenna apparatus capable of being used in the above frequency band, a solid teardrop-shaped omni-directional antenna apparatus is known. This antenna apparatus is comprised of a combination of a conical hole structure formed on a ground substrate, and a spherical body disposed on the conical hole structure in an inscribed manner (see Shin-Gaku Technical Report WBS 2003-12, 2003; Reference 2).
Generally, an antenna apparatus is a device for emitting electromagnetic waves carrying signals supplied to the antenna apparatus (transmission) or conversely for taking in and detecting external electromagnetic waves from outside (reception). To transmit the signal supplied to the antenna apparatus with the desirable efficiency, it is generally necessary to match the characteristic impedance of a waveguide connected to an antenna element with the input impedance of the antenna element. When the impedance of the waveguide is matched with the impedance of the antenna element, the signal supplied to the antenna element from the waveguide can be effectively emitted as electromagnetic waves. In contrast, when the impedance of the waveguide is mismatched with the impedance of the antenna element, a portion of the signal supplied from the waveguide is reflected by the antenna element, and the strength of the emitted electromagnetic waves is likely to decrease. Accordingly, the efficiency is reduced. It is known that such reflection of the signal occurs due to an abrupt change in the electromagnetic-field distribution attendant on a discontinuity in the shape of a conductor.
The antenna apparatus disclosed in Reference 1 is a resonant antenna apparatus, i.e., an antenna apparatus that is constructed to be used in a narrow band. In this antenna apparatus, the distance (i.e., the slot 2) between a side portion of the T-shaped conductor and an end portion of the waveguide is adjusted so as to effect desired the impedance matching between the antenna element and the waveguide. Such a method is often used when the impedance matching is carried out in a narrowband antenna apparatus.
However, if that matching method is applied to an antenna apparatus required to have the frequency characteristic in a broad band, an abrupt change in the electromagnetic-field distribution due to the discontinuity of its waveguide is likely to appear at some frequencies. It hence becomes difficult to achieve impedance matching in a broad band.
In contrast, the solid antenna apparatus disclosed in Reference 2 shows the impedance matching characteristic in a broad band. However, its size and weight are relatively large, and hence its utility is limited. Therefore, it is at present difficult to obtain an antenna apparatus that is relatively small in size and yet usable in a relatively wide frequency range.